Mantis sometimes send very long e-mails (more than 50k characters), especially for old issues with many notes.
In our compagny the SMTP server does not accept those long e-mails and this makes the whole Mantis getting stuck.
I saw that some other issues (0008885, for example) where due to e-mail not sent. This may be the cause.

I made a patch which adds a configuration option which allow to limit the e-mail size. This patch is made with standard diff (to be applied with patch -p1) against version 1.2.3. Sorry, I did not have time to set up git correctly.

However, till then if we want a quick fix for this, I would rather have a fix that would avoid the attachment getting too large by limiting the max number of notes, rather than the max number of characters. I would expect that having lots of notes is the reason why email notifications get too large and fail. We should also make sure that we are keeping the last N notes independent of how they are ordered based on configuration (new to old or old to new).

OK, so we seem to agree on the approach of having a limit on the number of issues. For example, 20?

I question the need for a per user configuration for this option. Should we deprecate this for 1.3.x+?

If we end up changing to the new notification model proposed, then both becomes unnecessary. I wonder if we just don't do anything here or just do a non-configurable limit to avoid adding a config and then deprecating it later.

just change the default from 0 to (whatever)
This will not help that much.
It works only for users that are created after the change.
Existing users will go on using their stored setting in mantis_user_pref_table.